Conventionally, the IEEE 1394 high speed serial bus, referred to below as 1394 bus, was used. By employing the 1394 bus, a plural number of electronic equipment in a household can be interconnected to form a household network.
Should a routine electrical cable be used as this 1394 bus, attenuation becomes severe such that limitations are imposed on the usable range. The standards for an electrical cable provide that the longest transmission range be 4.5 m. In such case, even interconnecting an equipment mounted on a wall of a room about 5.4 m2 in size to another equipment mounted on the opposite wall of the same room along the wall is rendered difficult.
If in transmitting/receiving data using an optical fiber, a preset bit error rate (BER) is to be realized, it is necessary to provide for a preset signal to noise ratio (S/N ratio). When it is checked whether or not a preset bit error rate (S/N ratio) can be realized using an optical fiber, a system shown for example, in FIG. 1 is usually presupposed. In this system, if the volume of light transmitted using the optical fiber is at a level H or at a level L, such states are verified to be a logical level H and a logical level L, respectively. In actuality, the amount of light received is changed analogically, so that, with a median decision level between the level H and the level L (H+L)/2 being a decision level D, the state of the volume of the received light being larger than D and the state of the volume of the received light being smaller than D are verified to be the logical 1 and the logical 0, respectively.
Conventionally, a bit error rate BER was calculated from the following equations, assuming that a Gaussian noise was superposed on each of the levels H and L:
                    BER        =                ⁢                                            1              2                        ⁢                                          ∫                                  -                  ∞                                D                            ⁢                                                1                                      σ                    ⁢                                                                  2                        ⁢                                                                                                  ⁢                        π                                                                                            ⁢                                  exp                  ⁡                                      (                                          -                                                                                                    (                                                          H                              -                              x                                                        )                                                    2                                                                          2                          ⁢                                                      σ                            2                                                                                                                )                                                  ⁢                                  ⅆ                  x                                                              +                                                ⁢                              1            2                    ⁢                                    ∫              D              ∞                        ⁢                                          1                                  σ                  ⁢                                                            2                      ⁢                                                                                          ⁢                      π                                                                                  ⁢                              exp                ⁡                                  (                                      -                                                                                            (                                                      x                            -                            L                                                    )                                                2                                                                    2                        ⁢                                                  σ                          2                                                                                                      )                                            ⁢                              ⅆ                x                                                                            =                ⁢                              ∫            D            ∞                    ⁢                                    1                              σ                ⁢                                                      2                    ⁢                    π                                                                        ⁢                          exp              ⁡                              (                                  -                                                                                    (                                                  x                          -                          L                                                )                                            2                                                              2                      ⁢                                              σ                        2                                                                                            )                                      ⁢                          ⅆ              x                                                              =                ⁢                              ∫                                          H                -                L                                            2                ⁢                σ                                      ∞                    ⁢                                    1                                                2                  ⁢                  π                                                      ⁢                          exp              ⁡                              (                                  -                                                            y                      2                                        2                                                  )                                      ⁢                          ⅆ                              y                ⁢                                                                  [                                  y                  =                                                            (                                              x                        -                        L                                            )                                        /                    σ                                                  ]                                                                            =                ⁢                              1                                          2                ⁢                π                                              ⁢                                    ∫              Q              ∞                        ⁢                                          exp                ⁡                                  (                                      -                                                                  y                        2                                            2                                                        )                                            ⁢                              ⅆ                                  y                  ⁢                                                                          [                                      Q                    =                                                                  (                                                  H                          -                          L                                                )                                            /                                              (                                                  2                          ⁢                          σ                                                )                                                                              ]                                                                        
In the above equations, BER is calculated as a ratio to the entire area of an area where the skirts of the Gaussian distribution of the level H overlap with those of the level L.
If, in the above equations, H−L is a signal level S and the noise is 6, the S/N ratio is related with the value Q by the following equation:S/N ratio=2Q
The relation between the bit error rate BE and the Q value is as shown in FIG. 1, wherein the ordinate and the abscissa denote BER and the Q-value, respectively.
In the 1394 bus, 1E−12 (=1×10·12) is required for the bit error rate. The corresponding Q-value is approximately 7 from FIG. 2. If Q=7 is re-calculated as S/N ratio, the result is 14, that is 11.5 dB. In many other communication systems, 1E−9 is required for the bit error rate. The corresponding Q value is 6, with the S/N ratio being 12, that is 10.8 dB.
Up to now, it was retained to be difficult to realize such bit error rate BER=1E−12 on a sole optical fiber by full duplex communication. The full duplex communication means the communication employing substantially the same wavelength for both the transmitting and receiving sides to effect light propagation for transmission and that for reception simultaneously.
For example, as shown in FIG. 3, the light of a preset wavelength is generated from a light transmitting device 2 of an equipment 1 so as to be transmitted over an optical fiber 21 to an equipment 2 for reception by a light receiving device 13 of the equipment 2. On the other hand, a light emitting device 12 of the equipment 2 emits light of substantially the same wavelength as that of the light generated by the light transmitting device 2 and transmitted over the optical fiber 21 for reception by a light receiving device 3 of the equipment 1.
If, on the side equipment 1, the light receiving device 3 receives only the light generated by the light emitting device 12 of the equipment 2, the equipment 1 is able to perform transmission/reception substantially simultaneously as the equipment 2.
In actuality, the light generated by the light emitting device 12 of the equipment 2 may leak directly into the light receiving device 3, may be reflected by an end face towards the side equipment 1 of the optical fiber 21 (proximate end face) so as to be received by the light receiving device 3, or may be reflected by an end face towards the equipment 2 of the optical fiber 21 (distal end face) so as to be received by the light receiving device 3. That is, the light receiving device 3 receives the return light of the light generated by the light emitting device 12 (stray light), in addition to the light generated by the light emitting device 12. This stray light was thought to be a noise component. The result is that, if this stray light is present in an amount of 10% of the signal amount, the S/N ratio is 10 (=1/0.1). If calculated as Q value, the S/N ratio is 5 (10/2). As may be seen from FIG. 2, the bit error rate for realizing Q=5 is approximately 1E−6.5. That is, should there be stray light of 10%, it is difficult to realize the bit error rate of 1E−9 required in many communication systems, to say nothing of the bit error rate of 1E−12 as required for a 1394 bus.
In view of the above-mentioned facts, a proposal has been made for providing an optical fiber 21A for transmission and an optical fiber 21B for reception, looking from the equipment 1, to effect transmission and reception simultaneously. This structure necessitates basically two optical fibers.
Also, a proposal has also been made for interconnecting the equipment 1 and 2 over a sole optical fiber 21 and for providing an optical system 31 or 41 for each of the equipment 1 and 2 to effect time-divisional communication.
The optical system 31 is basically configured as shown in FIG. 6. The opposite side optical system 41 is similarly configured, as shown in FIG. 6. This optical system 31 includes a prism 52 on a substrate 51. The light radiated from the light emitting device 2 is reflected by a surface 52A of the prism 52 and introduced through a lens 53 into the inside of the optical fiber 21 via end face 21A thereof so as to be transmitted to the side equipment 2. The light transmitted from the equipment 2 is radiated from the end face 21A of the optical fiber 21 to fall on an end face 52A of the prism 52 through lens 53 and is transmitted through the prism to fall on the light receiving device 3 formed on the substrate 51.
The optical systems 31, 41, configured as described above, separate the light for transmission, radiated from the light emitting device 12 and the light for reception transmitted from the optical fiber 21, by the prism 52.
However, pair of light radiated from the light emitting device 12 is not reflected from the surface 52A of the prism 52, but is introduced into the inside of the prism 52, so as to be received by the light receiving device 3. The processing for transmission and that for reception are carried out time-divisionally, as shown in the flowcharts of FIGS. 7 and 8.
That is, at step S1, the equipment 1 at step S1 verifies whether or not there is data for transmission. If there is no data for transmission, processing transfers to step S2 to check whether or not a use request signal for a line (optical fiber 21) has been received from the equipment 2. If there is no line use request signal from the equipment 2, processing reverts to step S1 to repeat the subsequent processing.
If it is verified at step S2 that the line use request signal has been received from the equipment, processing transfers to step S3 where the equipment 1 sets the reception mode. At step S4, it is verified whether or not a signal is being received from the equipment 2. That is, it is checked whether or not the line use request signal is being still received. If the signal is being received, waiting state is set until the signal ceases to be received. When the signal ceases to be received, processing transfers to step S5 where the equipment 1 advises the equipment 2 of the effect of setting of the reception mode.
If this notification is made, data is transmitted from the equipment 2, as will be explained subsequently. Then, the waiting mode is set at step S6 until data is received. On reception of the data, it is stored e.g., in a memory at step S7. At step S8, it is verified whether or not a data end signal is received and, if no signal has been received, processing reverts to step S6 to repeat subsequent steps.
If it is verified that the data end signal has been received at step S8, processing transfers to step S9 where the equipment 1 cancels the reception mode to revert to step S1 to repeat the subsequent steps.
If it is verified at step S1 that transmission data exist, processing transfers to step S10 where the equipment 1 outputs a line use request signal to the equipment 2. At step S11, the equipment 1 verifies whether or not reception mode setting notification has been received from the equipment 2. If the reception mode setting notification has been received, processing transfers to step S12 where the equipment 1 sets the transmission mode. At step S13, the equipment 1 sends data and, at step S14, it verifies whether or not data transmission has come to a close. If data transmission has come to a close, processing reverts to step S13 and, if otherwise, the processing reverts to step S13 to repeat subsequent steps.
If it is verified at step S14 that data transmission has come to a close, processing transfers to step S15 where the equipment outputs a data end signal to the equipment 2. At step S16, the equipment 1 cancels the transmission mode. Then, processing reverts to step S1 to repeat subsequent steps.
If it is verified at step S11 that the reception mode setting notification has not been received from the equipment 2, it may be conjectured that, at a timing of outputting the line use request signal at step S10 from the equipment 1 to the equipment 2, a line use request signal is also issued from the equipment 1, such that the two line use request signals overlap with each other with the result the equipment 2 has failed to receive the line use request signal output by the equipment 1. Thus, in such case, processing transfers to step S17 where the equipment 2 verifies whether or not the line use request signal has again been received. If the line use request signal has not been received, processing transfers to step S18 to verify whether or not a preset random time has elapsed since the outputting of the line use request signal. If this preset random time has not elapsed processing reverts to step S17 to execute subsequent steps repeatedly.
If it is verified at step S18 that the preset random time has elapsed, processing reverts to step S1 to execute subsequent steps repeatedly.
In similar manner, if the equipment 2 has output a line use request signal to the equipment 1, but no reception mode setting notification is sent from the equipment 1, the equipment 2 executes the processing of queuing until a preset random time elapses. In general, this preset random time is different from the preset random time as set for the equipment 1. If the preset time for the equipment 1 is longer than that for the equipment 2, the equipment 2 again sends a line use request signal when the equipment is at a standby state. In such case, the line use request signal is deemed to have been received at step S17, so that processing transfers to step S3 to execute the processing similar to that described above.
That is, in the present instance, transmission processing is initiated independently from both the equipment 1 and 2 and, failing a proper response from the counterpart side, the random standby state is set for both equipment to permit similar processing to be re-executed. Since this random time for the two equipment is usually different, the equipment with the shorter time setting is the first to transmit data.
Thus, in the conventional system transmitting or receiving data over a sole optical fiber, data is transmitted/received time-divisionally. As a result, a technical problem is presented that each equipment is unable to transmit data reliably at a preset timing.